Frequency-modulation tube with an ion getter pump



Oct. 28, 1969. YP.MEYERERIETAL 3,475,650

FREQUENCY-MODULATION TUBE WITH AN ION GETTER PUMP Filed Aug. 8. 1967 i.l: I

Y [flu/ mm RN EYS United States Patent US. Cl. 315- 5.38 5 ClaimsABSTRACT OF THE DISCLOSURE A frequency-modulation tube, with amagnetically bundled electron beam of high density, having alongitudinally extending interaction space, particularly travelling wavetubes with a delay line forming the interaction space, in which theelectron collector, for effecting a uniform anode dissipation, ismagnetically shielded by a soft iron shell, and, preferably along thelines of a depressed collector, has a lower potential with respect tothe delay line, and an ion getter pump having the ionization paththereof formed mainly in the interaction space by the electron beam, andcomprising at least one electrode, as an ion trap, disposed in theelectron collector and constructed of a vaporizable material capable ofgettering, such electrode comprising a metal plate forming the bottom ofthe electron collector and insulated therefrom, and having such apotential, such as cathode potential, that the ions formed in theelectron beam bundle strike it with considerable velocity, which platemay be provided with a multi-pronged centrally symmetrically disposedfork, whose mutual prong space is equal to or larger than the diameterof the collector entrance opening.

The invention relates to a frequency-modulation tube utilizing amagnetically focused electron beam of high density and with alongitudinally extending interaction space, especially travelling-wavetubes with a delay line forming the interaction space, wherein theelectron collector for a more uniform distribution of the anodedissipation, is magnetically shielded by a soft iron shell, and,preferably along the lines of a depressed collector, has a lowerpotential with respect to the delay line.

The design of the electron collector as a deceleration electrode inrelation to the delay line, along the lines of a depressed collector,primarily functions to reduce the anode dissipation and, therewith, toalso reduce the heat to be dissipated. The magnetic shielding of theelectron collector also serves to further improve the heat elimination,through the use of a soft iron shell, because elimination of thefocusing effect of the magnetic field results in such spreading of theelectron beam bundle within the collector that the electrons strike thecylindrical wall of the collector with a more uniform distribution, towhich wall, for example, the radiator plate of the associated coolingstructure is attached.

The problem underlying the invention resides in creating, in afrequency-modulation tube designed in the man- 3,475,650 Patented Oct.28, 1969 ner described, particularly travelling-wave tubes, a highvacuumpump along the lines of an ion getter pump, if possible without furtheradditional electrodes and thus primarily with the electrodes available,if necessary in slightly modified form, which pump during its operationcontinuously improves the vacuum of the frequencymodulation tube untilan approximately ultimate value is reached.

Ion getter pumps serving as a separate and, for practical purposes, afixed unit, as well as a directly attached part of an electron dischargesystem, are known as such in prior art. Their mode of operation is basedon the fact that residual gases are ionized and then transported andbound on an electrode provided with a gettering surface. To increase thegettering effect, a getter film is produced, usually by evaporation orvaporization, which is renewed or restored either constantly orperiodically. But an ionization path or stretch is required for suchion-drawing and ion-binding electrode, in which the residual gases aresufficiently ionized as a result of ionization by impact.

In the ion getter pump installations known in the prior art, specialelectron discharging spaces are provided, whose electron paths must beconsiderably extended through an additional strong magnetic field inorder to achieve a sufficient ionization, as otherwise the pumpingprocess simply breaks down.

Since for the pumping operation it is important that the ions formedduring the operation actually get to the ion trap which, if possible,therefore must take place simply and reliably, considerable difficultiesarise in the installation of the ion trap in a frequency-modulationtube. The insertion of the ion trap in the direct electron beam passage,where the ions formed in the electron beam would reach it in anefiicient manner, is not possible due to disturbances arising inconnection therewith. But in ion traps installed externally of theelectron beam it is, for. example, extremely difficult to laterallyconduct the ions formed in the electron beam out of the electron beampassage. However, through a series of extensive experiments it has beendetermined that by inserting an additional electrode, consisting of amaterial which is capable of gettering and can be vaporized through ionimpact, an ion getter pump can be created in a very simple way in theelectron collector itself without the slightest disturbance with respectto the operational mechanism of the frequency-modulation tube involved.

According to the invention, an important feature of such afrequency-modulation tube therefore resides in an arrangement in whichfor an ion getter pump having an ionization path formed by the electronbeam in the interaction space within the electron collector, there isprovided as an ion trap at least one electrode, of a vaporizablematerial capable of gettering, comprising a metal plate forming thebottom of the electron collector, which is insulated therefrom and hassuch a potential, particularly cathode potential, that the ions formedin the electron beam bundle strike it with considerable velocity. As theions formed in the electron beam strike the bottom plate insulated fromthe electron collector with considerable velocity, the electrodematerial is partially vaporized and travels to the wall of the electroncollector. At the same time, the electrons of the electron beam bundleare pushed aside or diverted to the cylindrical side wall of theelectron collector by the strong opposing field developed so that abetter distribution is effected. The bottom plate comprises a metalcapable of gettering, which is easy to vaporize through ion impact, e.g.titanium, and practically fills the entire cross section of the electroncollector. The insulation of the bottom plate with respect to theelectron collector can most simply be effected by the use of glassbeads, similar to those employed in the manufacture of leakproofcontainers, or by a metal ceramic compound. In the operation of such anelectrode arrangement, however, it appears that a certain portion ofsecondary electrons, especially of elastically reflected fast electrons,leaves the collector. These returning electrons are disturbing in thatthey produce a strengthening in a retrograde sense, e.g. in therespective travelling-wave tubes and, as a result, reduce thefront-to-back ratio. This, again, results in tendency for the tube toreadily oscillate. The reason for this lies in the fact, as wasdetermined through a series of experiments, that the potential linesdeveloping, as a result of the cathode potential of the insulated bottomplate, with respect to the cylindrical wall of the electron collector,partially extend at such a flat angle that a considerable portion ofions is reflected in the center of the electron beam bundle. Since,however, electrons actually entering centrally are still elasticallyreflected, a small fraction of disturbing electrons remains in suchcase.

However, this ion trap, provided with a centrally pointed pin, hasparticular importance for frequencymodulation tubes operating with ahollow electron beam, because all electrons of such an electron beamstrike on a strongly diagonally-deflecting field in the electroncollector. The ions carried along strike mainly on the central pin andvaporize its material. A pronounced elimination of the defect occurringin a full beam due to elastically reflecting interfering electrons isperhaps possible because the tip itself is not centrally arranged, i.e.it bends away from the center.

An advantageous further development of the ion trap resides, therefore,in a two or three-pronged fork, wherein the reciprocal prong distance islarger than or equal to the opening of the electron collector. In athree-pronged fork the entering electron beam is thereby divided intothree branch currents which strike the cylindrical side wall of thecollector and in this way cause a more or less, uniform heating of thecollector. Suitable shielding insures that the insulating bodyconnecting the electron collector and ion trap is not dampened by partsof the ion trap.

Further details of the invention will be explained with the aid of anembodiment illustrated purely diagrammatically in the drawings, inwhich:

FIG. 1 is a longitudinal section of an electron collector structureembodying the invention; and

FIG. 2 is a diagram indicating the form of the beam bundle and thevoltage appearing in the collector structure.

, For the sake of simplicity all parts not contributing to understandingthe invention, e.g. the important parts of the frequency-modulation tubeinvolved, are omitted.

The electron beam bundle of the associated frequencymodulation tubeinvolved, not illustrated is designated in FIG. 1 by the referencenumeral 1, and illustrated at the moment when it is leaving theinteraction space and entering the electron collector 2, which isconcentrically surrounded by a soft iron cylinder 3 forming a shield forthe electron collector with respect to the magnetic field and therebyenables the spreading of the electron beam bundle within the collector.In the electron collector itself there is provided, instead of anotherwise conventional bottom insulated as an ion trap, a bottom plate 4secured by a ceramic ring 7 to the collector 2 on which a threeprongedfork 5 is so mounted that the three Prongs there- 4 I of extend deeplyinto the collector space. Together they comprise the actual ion trap ofan ion getter pump and to this end are constructed of a material capableof gettering, which is easy to vaporize through ion impact. A biasingsource E is connected to plate 4 and collector 2 as shown. Due to thefact that the ion trap lies approximately on the potential of thecathode, the ions entering with the electron beam strike the fork 5,particularly the individual prongs 6 thereof, with considerablevelocity, thereby causing a vaporization of the metal thereof, which isdeposited on the cylinder wall of the electron collector 2, forming agetter film capable of gettering additional ions as well as gasmolecules. The ions formed in the electron beam bundle are thus mainlybound directly by the bottom plate with its cage-like fork, or else, bythe inner wall of the collector, provided with a getter film. Asapparent from FIG. 2, the electron beam bundle itself is strongly spreadout, e.g. in three component bundles, deflected upon the collector wall,while the electrons are subjected to corresponding transverse fields asa result of the potential course produced by the fork prongs. Althoughthe collector, along the lines of a depressed collector, has a smallerpositive potential than the delay line, not illustrated, disposed infront of it, and which forms the interaction space, as a result of thevery pointed potential barriers, developed by the prongs, and theirsteep sides only few electrons leave the electron collector aselastically reflected fast electrons. Reflected electrons are avoidedbecause the opening of the entrance diaphragm is so dimensioned that thefork tips are just covered thereby. In the practical development of theelectron collector, together with the ion trap, the specific form of thecage-like structure of the bottom plate can be varied considerably.

The arrangement described has considerable advantages with respect topreviously known arrangements. As an ionizing means, i.e. fortransforming the residual gases into ions for purposes of a guidedconduction it is effective with respect to entire electron beam bundleof high density and current intensity of the frequencymodulation tubeinvolved, so that there can be no danger whatever of a breakdown in theion supply. Through the absorption of the ions in the interior of theelectron collector an ion flow towards the collector occurs along theelectron beam, so that a diffusion of the ions takes place inside theelectron beam toward'the collector and therewith to the ion trap. As aresult of the special cagelike design of the ion trap a relativelyuniform distribution of electrons on the collector wall is achieved andthe discharge of elastically reflected fast electrons is considerablylimited, if not entirely prevented.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

. We claim:

1. In a frequency-modulation tube such as a travelling wave tube havingan electron beam of-high density which is bunched in an interactionspace, an anode comprising a cylindrical electron collector formedwithan electron receiving orifice into which the electron beam isreceived, an ion trap mountedin said cylindrical electron collector andcomprising a conductive plate of material capable of gettering mountedin said collector at the end away from said electron receivingorificeand insulated therefrom, a multiple pronged fork attached to saidconductive plate with the free ends of said prongs extending toward saidelectron receiving. orifice and lying in a circle and biasing meansconnected to said cylindrical electron collector and said ion trap tobias them so that no elastically reflected secondary electrons will passout of the electron collector through said orifice.

2. In a frequency-modulation tube according to claim 1 wherein saidconductive plate is formed of titanium and is circular in shape andsubstantially extends across the inside of said cylindrical electroncollector.

5 3. In a frequency-modulation tube according to claim 1 wherein saidconductive plate is attached to said cylindrical electron collector byceramic insulation material. 4. In a frequency-modulation tube accordingto claim 1 wherein the circle in which the free ends of the prongs 5 lieis a larger circle than the electron receiving orifice of the electroncollector.

5. In a frequency-modulation tube according to claim 1 comprising ashielding cylinder mounted about said cylindrical electron collector.

6 References Cited UNITED STATES PATENTS 3,328,628 6/1967 Yasuda et al.315-538 X 3,368,102 2/1968 Saharian 315-538 X JAMES W. LAWRENCE, PrimaryExaminer R. F. HOSSFELD, Assistant Examiner US. Cl. X.R.

